The ability of numerous bacteria to synthesize and accumulate a polymer of .beta.-hydroxybutyric acid (polyhydroxybutyrate, PHB) as an energy storage compound has long been recognized. The most commonly found compound of this class is poly(D(-)-3-hydroxybutyrate). However, some microbial species accumulate copolymers, which in addition to hydroxybutyrate, may contain longer chain hydroxyalkanoates. These co-polymers are referred to as polyhydroxyalkanoates (PHAs).
Interest has focused on PHAs because these biopolymers are thermoplastics and the physical properties of some PHAS resemble the properties of petro-chemically-based polymers such as polyethylene and polypropylene. However, unlike petro-chemically-based polymers, PHAs are both biocompatible and biodegradable. Certain species of bacteria have the ability to excrete enzymes and degrade PHAs. Because of the prevalence of these bacterial species in many natural environments, PHA is rapidly degraded in the soil and activated sludge. A factor making PHA even more attractive is that microbial PHA production utilizes renewable resources as starting materials, not petroleum. Thus, PHA is of interest as a renewable source of biodegradable thermoplastic.
While PHAs have been of general interest because of their biodegradable nature, their actual use as a plastic material has been hampered by their thermal instability. Industrial production of copolymers of hydroxybutyrate and hydroxyvalerate (PHB/V) from large-scale cultivation of bacteria began in 1982. The PHB/V produced in this way was marketed by ICI plc under the trade name Biopol. PHB/V is a thermoplastic having a high degree of crystallinity and a high melting temperature. As a result, PHB/V becomes unstable and degrades at elevated temperatures near its melting temperature. In addition, PHB/V has mechanical problems such as poor flexibility and poor impact resistance. Due to this thermal instability, and PHB/V's poor mechanical properties, commercial applications of PHB or PHB/V have been extremely limited. If these problems were overcome, PHAs could be utilized for many applications. For example, PHA could be used to make medical materials such as surgical thread or bone setting materials, hygienic articles such as diapers or sanitary articles, agricultural or horticultural materials such as multi films, slow release chemicals, fishery materials such as fishing nets and packaging materials.
Efforts to improve the mechanical qualities of PHAs have focused on altering monomer composition. The physical and mechanical properties of PHAs, such as stiffness, melting temperature, extension to break, and resistance to organic solvents, can change considerably as a function of the monomer composition. For example, melting temperature decreases as the level of monomers with greater than five carbons is increased. Studies of polyhydroxy-alkanoate production in Ralstonia eutropha (Alcaligenes eutrophus) have shown that when the bacteria are cultivated in a medium with carbohydrates such as glucose or fructose as a carbon source, only PHB is accumulated. However, when both carbohydrate and proprionic acid are provided as carbon sources, the bacteria accumulates copolymers of 3-hydroxyvalerate and 3-hydroxybutyrate (Holmes, P. A., Phys. Technol. 16:32-36 (1985):Holmes, P. A., Wright, L. F. and Collins, S. H. European Patents 0 069 497, January 1983 and 0 052 459, December 1985).
Because the properties of PHAs appear to improve with addition of 3-hydroxyacyl monomers with chain length longer than five carbons, efforts have focused on how to increase incorporation of medium chain heteroalkly units (C6 and above). However, these efforts have been made more difficult because many of the microbial strains which make PHAs make only PHB or only PHAs with short chain monomer units (C3 and C4). For example R. eutropha, until recently, was thought to only be capable of incorporating chain lengths up to C5. Anderson et al. "Biosynthesis and composition of bacterial poly(hydroxyalkanoates)", Int. J Biol. Macromol, Vol. 12, pp 102-105). One approach to this problem had been to genetically engineer microorganisms which are able to incorporate longer chain lengths. Another approach has been to affect copolymer composition by altering the carbon source utilized by the microorganism.
However, the improvements in the quality of the PHAs have been limited. For example, a copolymer comprising 3-hydroxybutyrate (3HB) and 3-hydroxyvalerate units (3HV) produced in Ralstonia eutropha is disclosed in EP 0 440 165 A2. The introduction of the 3HV component into the copolymer lowers the crystallinity and improves the flexibility. However, the copolymer still has problems in that the thermal resistance is not optimal for practical use because the melting temperature is such that extensive decomposition occurs during thermal melt molding. Therefore, this copolymer is not commercially used.
A copolymer has been described which contains small amounts of 3-hydroxycaproate or hexanoate (C6)(2.2%) or 3-hydroxyheptanoate (C7)(1.9%) units in R. eutropha. However, no C8 incorporation was reported. Ulmer et al. "The Bacterial Synthesis of Functional Poly .beta.-hydroxyalkanoates", Polymer Preparation, Vol. 30, No. 2, pp 402-403 (1989).
Another report utilizing A. eutrophus, discloses greater incorporation of C6 (6%) into the PHA copolymer. However, despite the use of C6 to C9 fatty acids as the carbon substrate, no C8 incorporation was reported. Volova et al. "Biosynthesis of Heteropolymeric Polyhydroxy-alkanoates by Chemolithoautotrophic Bacteria", Microbiology, Vol. 67, No. 4 pp. 420-424 (1998). In addition, these copolymers are not ideal in that the melting temperature is about 180.degree. C., and thermal decomposition would be expected to occur during thermal melt molding. Therefore, this copolymer would not be commercially useful.
To be commercially useful, a thermoplastic copolymer must have certain properties such as flexibility and moldability. At the same time it is desirable that the copolymer be biodegradable and be produced from a renewable source such as PHA which is microbially produced. Thus, there exists a need for a PHA copolymer which incorporates medium length hydroxyalkyl monomers and for a process for making such a PHA.